Electron-Cloud Effect
Encyclopedia
The electron-cloud effect (ECE) is a phenomenon associated with particle accelerators.
s disturb stray electrons already floating in the tube, and bounce or slingshot the electrons into the wall. These stray electrons can be photo-electrons from synchrotron radiation or electrons from ionized gas molecules. When an electron hits the wall, the wall emits more electrons due to secondary emission
. These electrons in turn hit another wall, releasing more and more electrons into the accelerator chamber.
accelerations, where electrons are attracted and slingshot into the walls at variable incident angles. Negatively charged electrons liberated from the accelerator walls are attracted to the positively charged beam, and form a "cloud" around it.
The effect is most pronounced for electrons with around 300eV
of kinetic energy
- with a steep drop-off of the effect at less than that energy, and a gradual drop-off at higher energies, which occurs because electrons "bury" themselves deep inside the walls of the accelerator tube, making it difficult for secondary electrons to escape into the tube.
The effect is also more pronounced for higher incidence angles (angles farther from the normal
).
Electron cloud growth can be a severe limitation in bunch currents and total beam currents if multipacting occurs. Multipacting can occur when the electron cloud dynamics can achieve a resonance with the bunch spacing of the accelerator beam. This can cause instabilities along a bunch train and even instabilities within a single bunch, which are known as head-tail instabilities.
, the vacuum pipe which contains the positron ring has a wire coiled around its entire length. Running a current through this wire creates a solenoid
al magnetic field
which tends to contain the electrons liberated from the beam pipe walls.
Retarding field analyzers are local grids in the chamber wall that allow some of the cloud to escape. These electrons can be filtered by an electric field and the resultant energy spectrum can be measured. Retarding field analyzers can be installed in drift regions, dipoles, quadrupoles, and wiggler magnets. A limitation is that retarding field analyzers measure only local cloud, and because they measure current, there is inherently some time averaging involved. The RFA can also interact with the measurement it is taking through secondary electrons from the retarding grid being expelled from the RFA and being kicked back into the device by the beam.
Witness bunch studies measure the tune shift along successive bunches in a train and in a witness bunch that is places at varying locations behind the train. Since tune shift is related to the ring-averaged central cloud density if the tune shift is known the central cloud density can be calculated. An advantage of witness bunch studies is the tune shifts can be measured bunch by bunch and so the time evolution of the cloud can be measured.
The vacuum chamber in an accelerator can be used as a waveguide for radio-frequency transmission. Transverse-electric waves can be propagated in the chamber. The electron cloud acts as a plasma and causes a density dependent phase shift in the RF. The phase shift can be measured as frequency sidebands which can then be converted back into a plasma density.
Explanation
Electron clouds are created when accelerated charged particleCharged particle
In physics, a charged particle is a particle with an electric charge. It may be either a subatomic particle or an ion. A collection of charged particles, or even a gas containing a proportion of charged particles, is called a plasma, which is called the fourth state of matter because its...
s disturb stray electrons already floating in the tube, and bounce or slingshot the electrons into the wall. These stray electrons can be photo-electrons from synchrotron radiation or electrons from ionized gas molecules. When an electron hits the wall, the wall emits more electrons due to secondary emission
Secondary emission
Secondary emission in physics is a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles. The primary particles are often charged particles like electrons or ions. If the secondary...
. These electrons in turn hit another wall, releasing more and more electrons into the accelerator chamber.
Exacerbating factors
This effect is especially a problem in positronPositron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron...
accelerations, where electrons are attracted and slingshot into the walls at variable incident angles. Negatively charged electrons liberated from the accelerator walls are attracted to the positively charged beam, and form a "cloud" around it.
The effect is most pronounced for electrons with around 300eV
Electronvolt
In physics, the electron volt is a unit of energy equal to approximately joule . By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt...
of kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
- with a steep drop-off of the effect at less than that energy, and a gradual drop-off at higher energies, which occurs because electrons "bury" themselves deep inside the walls of the accelerator tube, making it difficult for secondary electrons to escape into the tube.
The effect is also more pronounced for higher incidence angles (angles farther from the normal
Surface normal
A surface normal, or simply normal, to a flat surface is a vector that is perpendicular to that surface. A normal to a non-flat surface at a point P on the surface is a vector perpendicular to the tangent plane to that surface at P. The word "normal" is also used as an adjective: a line normal to a...
).
Electron cloud growth can be a severe limitation in bunch currents and total beam currents if multipacting occurs. Multipacting can occur when the electron cloud dynamics can achieve a resonance with the bunch spacing of the accelerator beam. This can cause instabilities along a bunch train and even instabilities within a single bunch, which are known as head-tail instabilities.
Proposed remedies
A few remedies have been proposed to deal with this, such as putting ridges in the accelerator tube, adding antechambers to the tube, coating the tube to reduce the yield of electrons from the surface, or creating an electric field to pull in stray electrons. At the PEP-II accelerator at SLACStanford Linear Accelerator Center
The SLAC National Accelerator Laboratory, originally named Stanford Linear Accelerator Center, is a United States Department of Energy National Laboratory operated by Stanford University under the programmatic direction of the U.S...
, the vacuum pipe which contains the positron ring has a wire coiled around its entire length. Running a current through this wire creates a solenoid
Solenoid
A solenoid is a coil wound into a tightly packed helix. In physics, the term solenoid refers to a long, thin loop of wire, often wrapped around a metallic core, which produces a magnetic field when an electric current is passed through it. Solenoids are important because they can create...
al magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
which tends to contain the electrons liberated from the beam pipe walls.
Measurement techniques
There are many different ways of measuring the electron cloud in a vacuum chamber. Each one gives insight into a different aspect of the electron cloud.Retarding field analyzers are local grids in the chamber wall that allow some of the cloud to escape. These electrons can be filtered by an electric field and the resultant energy spectrum can be measured. Retarding field analyzers can be installed in drift regions, dipoles, quadrupoles, and wiggler magnets. A limitation is that retarding field analyzers measure only local cloud, and because they measure current, there is inherently some time averaging involved. The RFA can also interact with the measurement it is taking through secondary electrons from the retarding grid being expelled from the RFA and being kicked back into the device by the beam.
Witness bunch studies measure the tune shift along successive bunches in a train and in a witness bunch that is places at varying locations behind the train. Since tune shift is related to the ring-averaged central cloud density if the tune shift is known the central cloud density can be calculated. An advantage of witness bunch studies is the tune shifts can be measured bunch by bunch and so the time evolution of the cloud can be measured.
The vacuum chamber in an accelerator can be used as a waveguide for radio-frequency transmission. Transverse-electric waves can be propagated in the chamber. The electron cloud acts as a plasma and causes a density dependent phase shift in the RF. The phase shift can be measured as frequency sidebands which can then be converted back into a plasma density.